The accurate measurement of time delay (or range delay) is important for many diverse applications. Most modern telecommunications systems, including cellular telephones, must be synchronized to less than one microsecond (usec). Satellite based radio frequency ranging systems (such as Global Positioning Systems) require time resolutions and synchronizations to less than 50 nanoseconds (nsec). Laser altimeter systems may require time resolutions of 1 nsec or better. New and existing applications in communications and remote sensing may substantially benefit from modulation methods that may reduce timing errors to one nsec or less.
Pseudo random noise (PN) code modulation is a signal type widely used in telecommunications, ranging, and altimetry applications. Presently, amplitude modulated PN coded systems use a pulse modulation pattern with an average 50% duty cycle, where the number of zeros and ones are approximately equal, and the width of the transmitted signal pulses (the “1” pulses) are equal to the bit period. PN code modulation may also be impressed on the phase, frequency, polarization, or pointing direction of the transmitted carrier. For amplitude modulated signals, the transmitter peak power is twice the average power. The receiver's timing resolution is proportional to the bit period divided by the receiver signal to noise ratio (SNR).
In most applications, there is a substantial benefit to improving the time resolution. For a PN coded signal with fixed receiver SNR, one has to increase the bit rate and decrease the bit period to improve the time resolution. For example, one must double the bit frequency to reduce the timing error by a factor of two. Thus, the use of conventional PN codes with a 50% duty cycle locks together many characteristics of the modulation, thereby limiting the capability to optimize performance. For an ex ample of the use of traditional pseudo noise (non-return-to-zero) code modulation in a laser ranging system see “Photon Counting Pseudorandom Noise Code Laser Altimeters”, SPIE Advanced Photon Counting Techniques, Paper 6771-23, September 2007, the entire contents of which are incorporated herein by reference.
There are several disadvantages to the fixed 50% duty cycle PN code modulation, as presently used: 1) For amplitude modulated signals, the transmitter peak power is fixed at twice the average power; 2) For amplitude modulated signals, the transmitter duty cycle is fixed at 50%; 3) For a given SNR at the receiver, the timing resolution can be changed only via the PN code's bit frequency or pulse rate; and 4) The duty cycle of the receiver integration time is 100%.
In laser applications using optical receivers, the amount of solar background noise and detector dark noise that is included in measurements is proportional to the duty cycle in the receiver's signal processing system. A 100% duty cycle means that all noise photons in the observation period contribute to the noise floor of the measurement. A reduction in the receiver duty cycle may reduce the impact of the noise (via time gating), may improve the SNR, and, therefore, may improve the timing resolution.